Does your DC have an EHCP? If so, what does it say in Section F about support? Section F is the part that is legally enforceable. If must lay out the quantifiable support DC requires in terms of hours, who's delivering the support, what training they must have etc.
If that is the case then the school MUST provide that support - even if it means getting agency staff in. That's what they do for my DS if his support staff are off sick.
You can email the senco/SLT with the wording of the relevant section in the EHCP to remind them of their duties. (Always put things in writing.)
If you haven't seen it there's some good info here:
https://www.ipsea.org.uk/what-should-be-in-the-sections-relating-to-education-sections-b-and-f
If Section F isn't clear and says things like: 'would benefit from' or 'should have access to' then sadly, it's much harder to argue for full time 1 to 1 support. You could ask for an emergency annual review (although I believe the LA doesn't have to agree to that).
School budgets are so stretched, if an EHCP isn't watertight then this, sadly is what can happen.
If your DC doesn't have an EHCP then you can apply for one even if the school isn't supportive. (This is what I had to do). Again, some good info here:
https://www.ipsea.org.uk/ehc-needs-assessments
Without an EHCP, it's so much harder to secure support. You can still ask to meet with the SENCO and explain how your DC is struggling and ask them what the plan is to ensure they can access the curriculum. Even without an EHCP, the school still has a duty to support DC - but the specifics can't be legally enforced.